1. Field of the Invention
The present invention relates to a semiconductor rod zone melting apparatus for manufacturing single-crystal semiconductors such as silicon, germanium, and the like, and compound semiconductors such as gallium phosphate, and the like by a floating-zone melting process (to be referred to as an FZ process hereinafter) and, more particularly, to a semiconductor rod zone melting apparatus wherein a metal piece is arranged close to or on at least one of upper and lower peripheral surfaces of an induction heating coil surrounding a floating zone of a semiconductor rod to be melted, and the space surrounding the floating zone can be varied by adjusting the relative positions of the coil and the metal piece.
2. Description of the Prior Art
A conventional apparatus for manufacturing a semiconductor single crystal is known. In this apparatus, a polycrystalline semiconductor rod as a starting material is held at the bottom of the upper shaft, and a single-crystal seed having a small diameter is held at the top of the lower shaft. One end of the semiconductor rod is melted by an RF induction heating coil surrounding the semiconductor rod and is nucleated on the seed crystal. Thereafter, a single-crystal portion having a smaller diameter than that of the seed crystal is grown at a high speed to obtain a dislocation-free crystal epitaxially from the seed, and at the same time, the semiconductor rod is started to zone melt while relatively rotating about and moving downwardly in the axial direction. In an apparatus of this type, it is preferable that the heating coil is arranged closer to a melting zone with a sufficiently small gap, so that electromagnetic energy is concentratively supplied from the coil to the melting zone. Therefore, like in a single-crystal nucleation process and a subsequent high-speed small-diameter single-crystal growth process, the effective inner diameter of the heating coil is preferably decreased with decreasing of the single crystal. In addition, if the diameter of the single crystal is increased as the melting zone is moved, the inner diameter of the heating coil is preferably increased accordingly. Such a means is further critical for manufacturing a large-diameter single crystal.
For this purpose, a technique (Japanese Unexamined Patent Publication (Kokai) No. 48-8801) has been proposed. In this technique, a multiple-turn main induction heating coil and a ring-shaped movable auxiliary induction heating coil are combined vertically with a small distance apart and both are independently powered. The auxiliary coil is gradually displaced laterally during operation, so that a space surrounding a floating zone defined by the coils may be kept to a minimum. However, in this technique, since RF currents which are respectively supplied to each of the parallel coils, have to precisely coincide in frequency with each other, there are found a great difficulty to achieve uniformity in a surrounding magnetic field around the semiconductor rod.
When a plurality of coils are vertically arranged, even with a small gap, the magnetic field excited by the coils on the semiconductor rod is broadened compared with that by a single coil. Therefore, it is difficult to concentrate a magnetic field to form a narrow floating zone, and hence difficult to operate on the larger diameter. This fact arouses a serious problem of being less able to make the single crystal with a large diameter dislocation free.
In order to eliminate the above drawbacks, another semiconductor rod melting apparatus is proposed (Japanese Patent Publication (Kokoku) No. 56-11674). In this apparatus, as shown in FIGS. 3(A) and 3(B), two arcuated metal pieces 102 and 103 which can be moved on and along the above or under surface of a flat ring-shaped main induction heating coil 101 with constant contact to keep electric conduction. The metal pieces 102 and 103 are moved closer to or away from each other in accordance with the diameter of a single-crystal to be manufactured, so that a space defined by the metal pieces 102 and 103 can be largely varied to keep strong coupling between them and the zone. However, with this prior art technique, the following problem remains unsolved.
More specifically, in an apparatus for manufacturing a semiconductor single crystal with zone-melting using a ring-shaped heating coil, a current flowing through the heating coil is increased as the diameter of a single crystal to be manufactured is increased, thus increasing an induction current generated on a peripheral surface of the semiconductor rod. However, since a large current needs large potential difference across the heating coil, electric discharge is easy to occur at moving electrical contacts of the metal pieces 102 and 103 with the coil 101. In general, since the single crystal is manufactured in an argon gas atmosphere as protection, the electric discharge is easy to occur.
The electric discharge not only adversely influences crystal quality but also causes coils 101, 102, and 103 which are made of copper or silver to be welded to each other.
In the prior art technique, a floating-zone surrounding space 104 is defined by the two arcuated metal pieces 102 and 103 having a smaller arc diameter on their inside than the inner diameter of the heating coil 101. For this reason, as the metal pieces 102 and 103 are separated from each other in accordance with an increase in diameter of a single crystal, as shown in FIG. 3(B), a slit space 104a defined between linear edges 102a and 103a is widened in such a manner that a stepped recess profile 104-104a were formed between the arcuated edges of the metal pieces 102 and 103 and the inner edge of the main induction heating coil 101. Thus, a nonuniform magnetic field is formed at the stepped recess 104-104a, and electromagnetic coupling with the semiconductor rod is reduced there. The nonuniform magnetic field causes floating-zone surrounding space 104a, and inhomogeneity in impurity concentration inside the single crystal since solidification at the growing front is repetitively interrupted.